(1) Field of the Invention
The present invention relates to coding method conversion apparatuses which code image information of moving pictures, and more particularly to a coding method conversion apparatus which performs variable length coding on quantized Discrete Cosine Transform (DCT) coefficients and outputs the coded data.
(2) Description of the Related Art
For technologies of coding moving pictures, coding efficiency has been improved. This allows mobile telephones to serve as television telephones having smooth moving images, capture high-quality moving pictures, and the like. With the development of coding technologies, conversion between different coding technologies has become necessary to share moving picture data between different apparatuses, to achieve long-length recording, and the like. For example, terrestrial digital television broadcast employs a moving picture coding technology called an MPEG-2 method. When TV programs provided by the above broadcast are to be recorded onto a Hard Disk Drive (HDD) recorder or the like, more programs can be recorded onto the recorder as longer-hour recording, by selecting a recording method with a coding rate lower than the MPEG-2 broadcast waves (including a change of a coding rate within the same method).
Coding apparatuses, which are represented by an apparatus which codes data by the MPEG-2 method (hereinafter, referred to as a “MPEG coding apparatus”), divide an image region into specific image blocks, perform orthogonal transformation and quantization on predictive residual signals which are generated by intra prediction or inter prediction, then perform variable length coding on the resulting signals in order to be outputted. Such coding apparatuses are liable to cause particular noise called block noise in decoded signals of the coded signals. In order to address this problem, a technology has been disclosed in which, when conversion between different coding technologies is to be performed, signals decoded from signals coded by the first coding device is applied with post filtering and block noise reduction, and then coded by the second coding unit (refer to Japanese Unexamined Patent Application Publication No. 2003-23640).
In the above technology, a coding type of the decoded signal is detected, and depending on whether the detected coding type is intra-prediction or inter-prediction, a strength of the post filtering is determined.